This invention relates to a stent for a vessel loaded in a vessel, such as blood vessel, lymphoduct, bile duct or urinary duct. The stent for the vessel is used for holding a constant state of the configuration of the vessel when the stent is loaded in the vessel.
Heretofore, if a constricted portion is produced in the blood vessel, such as artery, a balloon forming portion annexed to near the distal end of a catheter is introduced into the constricted portion in the blood vessel to from a balloon to expand the constricted portion to improve the blood stream, by way of performing a percutaneous blood vessel forming technique (PTA).
Meanwhile, it is known that, if the PTA is applied, the portion which once suffered from constriction tends to undergo re-constriction with a high probability.
For preventing this re-constriction, it is practiced to apply a tubular stent in the portion of the vessel treated with PTA. This stent is buried in an expanded state in a blood vessel 2 as shown in FIG. 11 to support the blood vessel 2 from its inside to prevent occurrence of re-constriction in the blood vessel 2.
In clinical cases in which a stent prepared by weaving a linear material of stainless steel in a mesh is introduced in the portion of the vessel treated with PTA, re-constriction occurred in a probability of approximately 15%.
For prohibiting this re-constriction, there is proposed in Japanese Laying-open publication Hei-5-502179 (WO91/01097) a stent prepared by polymer fibers containing a pharmaceutical capable of preventing the constriction from occurring.
Meanwhile, blood re-constriction after loading the stent occurs in a majority of cases at a stent end or beginning from the stent end.
The stent used in the inserted state in the blood vessel holds the blood vessel in the expanded state, so that it is designed to have tenacity sufficiently higher than that of the blood vessel. For example, the blood vessel has a Young""s modulus equal to approximately 3xc3x97107 pascal, whereas the stainless steel as a main material of the stent used for holding the blood vessel in the expanded state is approximately 3xc3x971011 pascal.
At this time, the blood vessel 2 is reduced in diameter at the portions in register with ends 1a, 1b of the stent 1 where the blood vessel 2 ceases to be supported by the stent 1. The portions of the blood vessel 2 supported by the ends 1a, 1b of the stent 1 represent stress-concentrated portions.
The blood vessel 2, especially the artery, perpetually performs pulsations to cause the blood to flow. The result is that the load due to the pulsations are repeatedly applied to the stress-concentrated portions of the blood vessel 2 supported by the ends 1a, 1b of the stent 1, so that the inner wall of the blood vessel 2 tends to be damaged by the ends 1a, 1b of the stent 1. Since the load is applied to the stress-concentrated portions of the blood vessel 2 supported by the ends 1a, 1b of the stent 1, there are produced injuries in the inner film or beginning from the inner film to the outer film of the blood vessel 2. If damaged, the blood vessel 2 recuperates the damaged portion as a reaction proper to a living body. In recuperating the damaged portion, the blood vessel 2 has its inner film multiplied excessively to cause the re-constriction.
It is therefore an object of the present invention to provide a novel stent for a vessel capable of eliminating the drawback of a conventional stent for the vessel.
It is another object of the present invention to provide a stent for a vessel which can positively prevent damages to the vessel whilst it positively keeps the vessel of a living body, such as a blood vessel, in an expanded state.
It is yet another object of the present invention to provide a stent for a vessel which can be inserted in position without producing stress-concentrated portions in the vessel.
For accomplishing the above object, the present invention provides a stent for a vessel, inserted in use into the vessel of a living body, including a tubular member constituting a passageway from one end to the opposite end thereof. The tubular member includes a main mid portion and low tenacity portions formed integrally with both ends of the main mid portion. The low tenacity portions is lower in tenacity than the main mid portion. The low tenacity portions are formed so as to have the Young""s modulus approximate to that of the vessel of the living body into which is inserted the stent.
The low tenacity portions are formed by stretching both ends of the tubular member for reducing the thickness thereat.
The low tenacity portions may also be formed by setting the surface density of both ends of the tubular member so as to be lower than that at both ends of the tubular member having a defined value. If the tubular member is prepared by knitting or weaving a fine metal wire or a polymer yarn, the low tenacity portions are formed by lowering the density of the meshes of knitting or weaving on both ends of the tubular member.
The low tenacity portions may also be formed by forming both ends of the tubular member of a material lower in Young""s modulus than the material making up the main mid portions.
If the tubular member is formed with a biodegradable polymer material, the stent holds its shape for a few weeks to a few months after insertion into the blood vessel. However, the stent can vanish in about several months after it biologically degrades in the vessel.
If the tubular member is formed by weaving or knitting a fine metal wire or a polymer yarn, the low tenacity portions can be formed on both ends of the tubular member by employing a soft linear material for these ends lower in strength than the linear material making up the main mid portion.